Table I. Thermochemical Properties of PAN and PBN PAN PBN Literature Estimated property at 298.15 K [Benson (1968) AHf" (g), kcal -57 ( 1 2 ) molL1 Buss (1958) Benson (1968) 26 ( 1 3 ) 35 (=t3) { Benson and Buss (1958) IBenson (1968) So (g), cal deg-1 98 ( 1 4 ) 116 ( 1 4 ) Benson and mol-' Buss (1958) AHvap", kcal 13 ( 1 2 ) 18 (+2) Bondi(1968) mol-' AHsublim", 17 ( 1 2 ) 23 ( 1 2 ) Bondi (1963) kcal mol-' (Bondi (1968) Cp" (liq), cal 42 (is) 60 ( 1 5 ) Shaw (1969) deg-I mol-' Calculated property at 298.1 5°K ASfo (g), cal -97 ( 1 4 ) deg-1 mol-' AGf" (g), kcal -28 ( 1 3 ) mol-' AHf " (liq), kcal - 70 (+3) mol-' AHf" (s), kcal -74 ( 1 3 ) mol-' 16 ( 1 6 ) ACp" [(liq) ( d l , deg-l mol-'
-117 ( 1 4 ) calcd +5 ( 1 3 ) calcd
-48 ( 1 3 ) calcd
- 53 ( 1 3 ) calcd 25 ( 1 6 ) calcd
tainties, in parentheses, for the properties we have estimated. A certain amount of arbitrariness has been exercised in the uncertainty assignments. To calculate ASf", we used values for the entropies of the elements in their standard states at 298.15'K, as tabulated by Wagman et al. (1968). The recent thermodynamic and thermochemical texts by Stull et al. (1969) and Cox and Pilcher (1970) were very helpful in providing data for use as auxiliary checks. It is interesting to note that, as a result of applying the estimation methods, we find that under equivalent conditions, PAN would form spontaneously from its elements (negative AGf"), and PBN would decompose to its elements (positive AGf "). Literature Cited
Benson, S. W., "Thermochemical Kinetics. Methods for the Estimation of Thermochemical Data and Rate Parameters," J. Wiley and Sons, Inc., New York, 1968. Benson, S. W., Buss, J. H., J . Chem. Plzys. 29, 546-72 (1958). Benson, S. W., Cruickshank, F. R., Golden, D. M., Haugen, G . R., O'Neal, H. E., Rodgers, A. S., Shaw, R., Walsh, R., Chem. Rev. 69,279-324 (1969). Bondi, A,, J. Chem. Eng. Data 8, 371-81 (1963). Bondi, A,, "Physical Properties of Molecular Crystals, Liquids, and Glasses," J. Wiley and Sons, Inc., New York, 1968. Cox, J. D., Pilcher, G., "Thermochemistry of Organic and Organometallic Compounds," Academic Press, Inc., New York and London, 1970. Shaw, R., J. Cliem. Eng. Data 14, 461-5 (1969). Stull, D. R., Westrum, E. F., Jr., Sinke, G. C., "The Chemical Thermodynamics of Organic Compounds," J. Wiley and Sons, Inc., New York, 1969. Wagman, D. D., Evans, W. H., Parker, V. B., Halow, I., Bailey, S. M., Schumm, R. H., NBS Technical Note 270-3, January 1968. Received for review October 12, 1970. Accepted December 26, 1970.
Calculation of Excess Air for Combustion Processes Using 0,-to-N, Ratio R. L. Miller and J. D. Winefordner Department of Chemistry, University of Florida, Gainesville, Fla. 32601
For most incinerators, it is possible to estimate the percentage of air in excess of the air needed for complete combustion of the fuel by simply measuring the ratio of O2to N z . This method is simpler and more reliable than the standard method of estimating excess air by measuring the percentage of coz.
I
n the analysis of incinerators and stack gases, it is desirable to have a convenient means for measuring the amount of air in excess of the air needed to burn the fuel completely-Le., stoichiometric combustion. In the Orsat analysis (Stern, 1968), information is provided that can be used to estimate the percentage excess air. From a brief ex444
Environmental Science & Technology
posure to stack sampling, it seemed apparent that for many incinerators, the ratio of O2to Nawould provide a better means for estimating the percentage of excess air where the composition of the fuel is variable and the incinerator temperature is not excessively hot, causing the Na in the air to oxidize. A theoretical comparison of the percent COa and 0 2 to NZ method is given below.
Comparison of Methods Percent COZMethod. The typical determination of excess air from the percentage of COz using the Orsat instrument has the following limitations : (a) the percentage excess air represented by a certain percentage COz varies with the fuel type; and (6) other acidic gases-eg., SOn, HC1, NO,, etc.typically present in stack gases interfere with the determination of C 0 2by NaOH absorption.
Some examples of the calculations and results using the percent COz method are I. STOICHIOMETRIC BURNING OF CELLULOSE (Ct3HloOs),
+ 6 + Ce.H1005
22.6 Nz
0 2
-+
6 COz
+ 5 H20 + 22.6 N2 +
The percentage of COSin this idealized case is simply [6/(22.6 6)]100% = 21.0%. This neglects the small deviation of the actual cellulose formula due to the terminal H’s and 0 ’ s . The ratio of O 2to Nz is 0.0%. 11. 50 % EXCESS AIR FOR BURNING OF CELLULOSE 33.8 Nn
+ 9 On + C6H10Oa
+
6 C02
+ 5 HzO + 33.8 N2 + 3
0 2
With the same type of calculation as above, the percentage C o nby volume for dry air is 14.0% for this second idealized case. The ratio of O2to Nzis 0.0886. 111. STOICHIOMETRIC BURNING OF METHANE CHI
+ 2 + 7.52 Nz + CO2 + 2 Hz0 + 7.52 Nz 0 2
For this idealized case, the percentage of C o n is 11.7%. The ratio of O2to Nnis 0.00. IV. STOICHIOMETRIC BURNING OF II-OCTANE CsHis
+ 12.5 + 47.0 Nz 0 2
-+
8 CO?
+ 9 HzO + 47.0 N2
For this idealized case, the percentage of COZis 14.5%. The ratio of 0 2 to Nzis 0.00. Therefore, in the above examples, the percentage of Con is not directly related to the percentage of excess air, but rather varies with composition of the fuel. For the case of the incinerators where the composition of the fuel is unknown or variable, the percentage of COZas a monitor for percentage of excess air loses much of its meaning. 02-to-N2Ratio Method. The ratio of O 2 to N2 should be independent of the fuel composition. The measurement of 0 2 and N 2 (and other gases inert to the Orsat instrument) are less likely to be interfered with than the measurement of Cog. The limiting assumption is that N 2 and other gases inert to the Orsat method and initially present in the air are neither consumed nor produced significantly by the combustion process; this assumption is probably valid for most incinerators. Some general examples of the calculations and results using the O2to N2 ratio are
Table I. Percentage of Excess Air Corresponding to Volume Ratio of 0 2 to N2 Excess air, 0 2 to Nz 77 volume ratio 0 0.0000 50 0.0886 100 0.133 150 0.160 a2 0.266
I. STOICHIOMETRIC CASE. According to the balanced chemical 10
2
+ 3.76 Nz + fuel
+0
O2
+ 3.76 Nz + products
reaction, the ratio of Ozto Nnis 0.00. 11. 50% EXCESSAIR CASE. According to the balanced chemical 1.50 Oz
+ 5.64 Nz + fuel
-+
0.5 O2
+ 5.64 N2 + products
reaction, the ratio of O2to N2is (0.5/5.64) 111. 100% EXCESS AIR CASE 2 O2
+ 7.52 Nz + fuel
+
1 O2
=
0.0886.
+ 7.52 Nz + products
According to the above chemical reaction, the On-to-Nz ratiois0.133. IV. INFINITE EXCESS AIR.For this limiting case, the On-to-Nz ratio is simply (1/3.76) = 0.266. In Table I, the percentage of excess air and the corresponding ratio of volume On to volume of N n is given. For cases where the volume of Oz is small and difficult to measure for allowable percentages of excess air, a larger sampling volume -eg., 200 ml-should be used. The ratio of O2to Nn, R , and the percentage excess air, P , are related by the equation P
R =
3.76 P
+ 376
Literature Cited Stern, A. C., “Air Pollution,” Vol. 3,2nd ed., Academic Press, New York, N. Y., 1968. Receivedfor review August 12, 1970. Accepted January 7, 1971
Volume 5, Number 5, May 1971 445
